Add helpers module with data structures, algorithms, and interactive demos

[Copilot]

Addresses request for reusable utilities to debug and solve LeetCode problems. Provides conversion functions, visualization tools, and common algorithms for binary trees and linked lists.

Implementation

Core modules:

• data_structures.py - Standard TreeNode and ListNode classes matching LeetCode definitions
• tree_utils.py - Bidirectional list↔tree conversion, visualization, tree properties
• linked_list_utils.py - Bidirectional list↔linked list conversion, node access utilities
• algorithms.py - BFS/DFS traversals, path finding, LCA, BST operations

Documentation:

• Interactive Jupyter notebooks with "Open in Colab" badges demonstrating all functions
• Comprehensive README with API reference and usage examples

Usage

from helpers import list_to_tree, print_tree, bfs_traversal, lowest_common_ancestor

# Create tree from LeetCode test case format
tree = list_to_tree([3, 5, 1, 6, 2, 0, 8])

# Visualize structure
print_tree(tree)
# Root: 3
# ├─ L: 5
# │  ├─ L: 6
# │  └─ R: 2
# └─ R: 1
#    ├─ L: 0
#    └─ R: 8

# Run algorithms
bfs_traversal(tree)  # [3, 5, 1, 6, 2, 0, 8]
lowest_common_ancestor(tree, 5, 1).val  # 3

# Convert back for submission
tree_to_list(tree)  # [3, 5, 1, 6, 2, 0, 8]

All functions handle edge cases (empty structures, single nodes) and maintain compatibility with LeetCode's format conventions.

Original prompt

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This section details on the original issue you should resolve

<issue_title>helper functions</issue_title>
<issue_description>it'd be nice to have some sample notebooks/code to help with debugging and solving these problems. for example:

• convert a list to a binary tree (and vice versa)
• convert a list to a linked list (and vice versa)
• visualize a binary tree
• define classes for some common data structures (linked list, binary tree)
• implement some classic algorithms-- tree search (BFS, DFS), find common ancestor in a tree, etc.

could add these as we go... 🤔 </issue_description>

Comments on the Issue (you are @copilot in this section)

@jeremymanning add a "helpers" folder, with .py files and organized by theme. then populate with relevant data structures and algorithms. and add some .ipynb notebooks (with an Open in Colab badge at the top) to demonstrate the functionality, along with a README.md file in the helpers folder with links to the relevant notebooks/files.

• Fixes helper functions #30

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